Resistive thermometers are temperature detecting or sensing devices that exploit the predictable change in electrical resistance of select temperature sensing materials with changing temperature. Such devices are generally characterized by a metallic element (i.e., a resistive element), and insulated electrical leads operatively linked thereto and extending therefrom. Resistance temperature detectors (RTDs) advantageously include a resistive element, i.e., a select temperature sensing material, comprised of a pure metal whose resistance increases with temperature, while thermistors advantageously utilize a resistive element comprised of a semiconductor whose resistance decreases as temperature increases.
In connection to RTDs, numerous types are known, for example and without limitation, platinum resistance thermometers (PRTs), standard platinum resistance thermometers (SPRTs), industrial resistance thermometer elements, industrial RTD probes, and flexible resistance thermometers. Moreover, RTD metallic elements comprise a variety of forms, with common styles including, but not limited to, unsupported wirewound, wirewound in a ceramic insulator, wire encapsulated in glass, and thin film.
As is well known, the metallic sensing element of an RTD is manufactured to have a specific electrical resistance at a specific temperature. Temperature is determined by measuring resistance, then using the RTD's resistance (R) versus temperature (T) characteristics, i.e., an R/T curve, to extrapolate temperature.
Platinum (Pt) is the most widely specified RTD sensing element type owning to its linear/near linear resistance response over a wide temperature range/ranges, stability, and standardization between manufacturers. Copper (Cu), nickel (Ni), and nickel-iron (Ni—Fe) RTD sensing element types are likewise known and utilized to a lesser extent, but nonetheless suitable for select applications, owing to sensing range limitations having origins in non-linearities in the temperature resistance profile/relationship, and wire oxidation potential in the case of copper.
Resistance/temperature characteristics for RTD sensing elements are well known, and subject of much documentation and a variety of industry standards. As a function of RTD sensing element material (e.g., Pt, Cu, Ni), known standards define resistance versus temperature behavior via non-linear mathematical models, e.g., Pt generally follows a modified Callendar-Van Dusen equation over the range −200 to 850° C., i.e., Pt's R/T curve is modeled using the modified Callendar-Van Dusen equation over the range −200 to 850° C. While the particulars are beyond the scope of the instant disclosure with regard to the mathematical models, reference is made to “Resistance Thermometry: Principles and Applications of Resistance Thermometers and Thermistors,” © 2008, Minco, MN, USA, incorporated herein by reference in its entirety.
Historically, RTDs have been specified by their resistance at zero degrees Celsius (0° C.), i.e., “R(0)”, and a temperature coefficient of resistance (TCR) “alpha” (α). The TCR, in the context of resistance thermometers, is oftentimes defined as the average resistance change per degree Celsius, over the range 0 to 100° C., divided by R(0); more simply, the TCR corresponds to the slope of the R/T curve. In one sense, TCR expresses the sensitivity of the resistive sensing element of the RTD. Although an RTD with a higher sensitivity is not necessarily more accurate, an attendant larger signal simplifies output electronics and is less susceptible to leadwire effects and electrical noise. Moreover, a larger resistance produces the same voltage output with less measuring current which limits self-heating of the element. Literature generally reports the following TCR values, Ω/Ω/° C.: Pt, 0.00375-0.003927; Cu, 0.00427; Ni, 0.00678-0.00672; Ni—Fe, 0.00518-0.00527. As is generally reported, RTDs are susceptible to three types of errors, namely, inherent device tolerance, gradients between the device and the medium to be sensed, and error introduced along the path of between the sensor and recorder/display/controller. As to inherency errors, conformity specifies the amount of resistance a device is permitted to depart from a standard mathematical model, e.g., the modified Callendar-Van Dusen equation for Pt. Conformity is characterized by two components, namely, a tolerance at a reference temperature, usually 0° C., and a tolerance at/on the slope, i.e., with regard to the TCR. Generally, an RTD conforms most closely to its R/T curve at the reference temperature, with the resistance “fanning-out” at temperatures above and below the reference temperature.
Owing to a variety of manufacturing factors, resistance shifts, i.e., departures in R(0) values, are commonplace with regard to the resistive temperature sensing element of an RTD. Moreover, the shift variability from one device to the other is just that, variable. Factory calibration checks and calibrations of such devices is thus general operating procedure.
As to heretofore known calibration approaches, resistive wire temperature sensors are commonly built with a high or excessive initial resistance, and arranged such that a portion may be selectively removed from the sensing area. Once removed, the wire pattern is then re-connected or united, as by fusing wire portions together or welding them together with a jumper material.
Alternately, resistive patterns are sputtered or etched onto a substrate using the same resistive material as the sensing element of the RTD, with strand removal or bridging selectively enabled to calibrate the sensor up or down to a target resistance. Using a pattern out of the same resistive material eliminates any altering of the sensing element's electrical properties.
Further still, resistive patterns may be made out of resistors of a dissimilar resistive material from that of the sensing element, with strand removal or bridging selectively pursued to suitably calibrate the sensor. Due to resistive material dissimilarity, and commensurate differences in TCR, such serial arrangement will alter the overall TCR of the RTD. This latter method is typically used for small calibration adjustments so as not to buffer the electrical properties of the RTD out of tolerance.
While heretofore approaches commonly satisfy the overall calibration objective, it is believed advantageous to provide an improved calibration component. More particularly, it is believed advantageous to provide means for calibration of the electrical resistance of a RTD with minimal influence or impact upon its TCR. Moreover, is believed advantageous to simplify the manufacturing of RTDs, and to allow consistent tighter calibration of RTDs over a greater operational range.